Programmable logic devices often include two separate power supply inputs. A system supply voltage, typically 3.3 V or 5 V, is provided to a first power supply input to provide current for read operations. Another voltage, typically 12 V or a voltage equal in value to the system supply voltage, is provided to a second power supply input. This voltage is provided for program and erase operations. A 12 V supply applied to the second power supply input will allow the programmable logic device to perform fast program and erase operations, while a voltage equal in value to the system supply voltage applied to this input will allow slower program operations. Typically, the 12 V supply is used to program the programmable logic device during production, thereby improving production throughput and minimizing production equipment costs. For typical in-system operations, 3.3 V or 5 V system supply voltage is applied to the second power supply input. In order to allow for fast programming during production and programming during normal in-system operation, a computer system manufacturer must implement system level circuitry to allow 12 V to be applied to the second power supply input during production and to cause the system supply voltage to be applied to the second power supply input during computer system operation. The system level circuitry may also include circuitry to provide a write-protection function by connecting the second power supply input to ground, thereby disallowing any write operations.
FIG. 1 shows a typical arrangement for a prior art programmable logic device 110 in a computer system. The programmable logic device 110 includes a system supply voltage input (VCC) 120 and a program and erase voltage input (VPP) 130. For this arrangement, VCC 120 is connected to VPP 130. VCC 120 and VPP 130 each receive a system supply voltage 140, which is typically 5 V, although 3.3 V is also typical. This implementation does not allow for fast programming, since no 12 V supply is provided. Also, no write protection is possible since VPP 130 is always equal to VCC 120, thereby enabling in-system programming and erase operations.
FIG. 2 depicts an additional typical arrangement for a prior art programmable logic device 110 in a computer system. As in FIG. 1, the programmable logic device 110 includes a VCC input 120 and a VPP input 130. For this implementation, a system supply voltage 140, typically 5 V or 3.3 V, is applied to VCC 120. A program and erase voltage 250, typically 12 V, is applied to VPP 130. VPP 130 is also connected to ground through a resistor 252. This implementation allows for fast production programming since VPP 130 receives 12 V. This implementation also provides write protection since when the programming and erase voltage 250 is not being applied to VPP 130, VPP 130 is left floating and is tied to ground through the resistor 252. This implementation has the disadvantage of not allowing in-system program and erase operations, since there is no provision for providing the system supply voltage 140 to VPP 130.
FIG. 3 illustrates an arrangement for a prior art programmable logic device 110 that allows for write protection, fast production program and erase operations, and in-system program and erase operations. For this arrangement, a supply voltage 140, typically 5 V or 3.3 V, is provided to VCC 120 and to a transistor 370. A system logic 360 is provided to enable transistor 370 when in-system program and erase operations are desired. When the transistor 370 is enabled, the system supply voltage 140 is delivered to VPP 130, thus enabling in-system program and erase operations. When the transistor 370 is not enabled, VPP 130 is normally tied to ground through resistor 252, thereby providing write protection by disabling programming and erase operations. With the transistor 370 disabled, it is also possible to apply a program and erase voltage 250 to VPP 130 during production, thereby enabling fast production program and erase operations. While this arrangement allows for fast production program and erase operations, in-system program and erase operations, and write protection, these features are available only at the extra cost of providing the system logic 360, the transistor 370, and the resistor 252.